Graphene is a monolayer of conjugated sp2 bonded carbon atoms tightly packed into a two-dimensional (2D) hexagonal lattice. It has been the subject of much research in recent years due to its unique electrical, optical, and physical properties. One of those properties is its high electron mobility, which has made it very desirable as the foundation of electronic devices such as transistors, capacitors, and sensors. The two-dimensional nature of graphene also makes it highly attractive for optoelectronic applications, while its tightly packed hexagonal structure makes it attractive for micro-mechanical device applications. See, e.g., R. Faccio, L. Fernández-Werner, H. Pardo, C. Goyenola, P. A. Denis, and Á. W. Mombrú, “Mechanical and Electronic Properties of Graphene Nanostructures,” Physics and Applications of Graphene—Theory, Dr. Sergey Mikhailov (Ed.), ISBN: 978-953-307-152-7 (2011); and F. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nature Photonics, Vol. 4, September 2010, pp. 611-622.
Creating such electronic devices requires patterning of the raw materials making up the device. For electronics and similar applications, the industry standard is to use an optical and chemical-based lithographic technique for pattering.
A variety of methods have been developed to pattern graphene.
Most conventional patterning of graphene involves the use of wet or dry chemicals applied to the surface as lithography resists to form a mask for patterning by etching.
However, unlike conventional three-dimensional (3D) materials which retain their bulk 3D properties when they are patterned to form electronic devices on their surface, graphene is in the form of a continuous two-dimensional surface having no three-dimensional properties, such that anything which interacts or attaches to its surface modifies its properties. Although this property of graphene makes it a good sensor, it makes it hard to chemically pattern graphene into devices, since the chemicals conventionally used for patterning can influence the properties of the graphene and so may not be suitable for patterning such a material.
For example, unlike with conventional 3D materials, residue from the resist cannot be cleaned from the graphene surface after patterning by O2 plasma ashing due to low selectivity between graphene and carbon-based polymers. The presence of such residue is a p-type compensator of the graphene and also increases carrier scattering, significantly reducing the graphene's potential when it is patterned into circuits and devices. Such resist residues also inhibit conformal metal deposition that leads to high and inconsistent graphene-metal contact resistance.
Other methods have attempted to pattern graphene by using a laser. Many of these methods, however, start with a material that is not graphene and transform it into patterned graphene using a laser, a thermal anneal, or other process, and the quality of the patterned graphene produced by these methods is usually much poorer than that of unpatterned large-area graphene.
For example, Han et al. use a laser to achieve a monolayer of graphene from multilayer graphene by heating to “thin” the graphene sheets. See G. H. Han, S. Chae, E. S. Kim, S. F. Güne, I. Lee, S. W. Lee, S. Y. Lee, S. C. Lim, H. K. Jeong, and M. S. Jeong, “Laser thinning for monolayer graphene formation: heat sink and interference effect,” ACS Nano 2011, 5, 263-268.
Zhou and Loh use a laser to locally modify graphene oxide (GO) and then use hydrazine gas or high-temperature annealing to create the graphene. See Y. Zhou and K. P. Loh, “Making patterns on graphene.” Advanced Materials 2010, 22, 3615-3620.
Zhou et al. use a focused laser beam to construct an extended area of micropatterned GO and reduced GO multilayers on quartz substrates. See Y. Zhou, Q. Bao, B. Varghese, L. A. L. Tang, C. K. Tan, C. Sow, and K. P. Loh, “Microstructuring of graphene oxide nanosheets using direct laser writing,” Advanced Materials 2010, 22, 67-71.
Wang et al. mention (but do not demonstrate) a means of converting graphane to graphene by a laser microstructuring system in a single chamber for simultaneous “grow-and-pattern” process. See Y. Wang, X. Xu, J. Lu, M. Lin, Q. Bao, B. Özyilmaz, and K. P. Loh, “Toward high throughput interconvertible graphane-to-graphene growth and patterning,” ACS Nano 2010, 4, 6146-6152.
Sokolov et al. demonstrate direct laser-induced reduction of graphite oxide into patterned graphene films. See D. A. Sokolov, K. R. Shepperd, T. M. Orlando, “Formation of graphene features from direct laser-induced reduction of graphite oxide,” Journal of Physical Chemistry Letters 2010, 1, 2633-2636; see also U.S. Patent Application Publication No. 2011/0318257 by D. A. Sokolov, K. R. Shepperd, T. M. Orlando, “Production of graphene sheets and features via laser processing of graphite oxide/graphene oxide,” (2011).
Zhang et al. use a femtosecond laser to locally reduce graphene oxide into patterned graphene. See Y. Zhang, L. Guo, S. Wei, Y. He, H. Xia, Q. Chen, H. Sun, and F. Xiao, “Direct imprinting of microcircuits on graphene oxides film by femtosecond laser reduction,” Nano Today 2010, 5, 15-20.
In all of these methods, either the graphene is damaged by the process or large areas of graphene or some other starting material must be produced and then processed to produce the desired patterns and properties. This increases the expense and complexity of graphene devices and reduces their yield. Thus, there is a need for a method for producing a localized pattern on graphene that does not introduce compensating impurities or scattering sites.